Modify Parse Tree

Question

I have an ANTLR ParseTree for an sql grammar.

Example :

My goal is to edit this tree so that i can delete all the middle (booleanExpression, predicated, valueExpression, primaryExpression ) nodes inbetween. I have explored visitor and listenors but they don’t generate the tree for me. And i'd like to not touch the grammar since its the official source one.

So how can i do it? Thanks

Answer 1

There is no such feature in ANTLR's API (removing/mutating the ParseTree). You'll have to walk the tree yourself and create a copy of the ParseTree and ignore certain sub-trees you do not want/need.

Answer 2

I’ve usually found the ParseTree to be too cumbersome (for exactly the reason you’ve shown). I don’t know of any automated way to alter the tree in memory. ( Could be an interesting project to attempt.)

I a couple of implementations I’ve written some generalized approach to make the process easier. The “best” one was where I wrote a small DSL to define the structure I wanted. It generated the classes as well as ANTLR style visitors and listeners for those. I then used a listener to transform the ANTLR ParseTree to my ideal tree, and wrote the rest of my code using that structure with that, much simpler, tree. You can have a listener/visitor generate a tree of your own design as an artifact of processing the ParseTree, but that’s as close as I’ve come.

It was actually a relatively minor effort to set that up early in the process and accounted for a quite small percentage of the overall effort of implementing the language (so, well worth the effort).